This report described studies on sodium, proton, potassium, calcium and other ion fluxes in relation to the control of intracellular and extracellular environents, acid-base balance and metabolism; their control by hormones, pathophysiological effectors and pharmacological agents; and how these regulatory systems are affected by age and by age-associated disease. The findings summarize investigations on: 1. Na+-H+ exchange activity decreased in the sensescent rat. 2. Mechanism of the renal response to an acid load in aging. 3. Thyroid hormone regulation of Na+-H+ exchange activity. 4. Biochemical characterization of the Na+-H+ exchange carrier. 5. Parathyroid hormone (PTH) regulation of cytosolic Ca2+ in proximal tubules. 6. Regulation of protein kinase C. 7. Pharmacological studies of the calcium-activated potassium channel in cultured medullary thick ascending limb cells.